This invention relates to an automatic reset circuit for providing a reset signal to one or more circuit elements, such as flip-flops, which are required to be reset when a power supply connected to these circuit elements is turned on.
In the field of digital circuitry, various flip-flops are used as circuit elements of a counter or a shift register. As well known in the art, a flip-flop has two stable states. For example, in an R-S flip-flop, a high level signal (H) is outputted when a set signal is received at its set input and a low level signal (L) is outputted when a reset signal is received at its reset input. A counter including such flip-flops performs counting by calculating the number of, for example, the high level signals during a desired period. In order to count correctly, the flip-flops must be set to their initial state or, in other words, must be reset at the beginning of the counting by a reset signal at the reset inputs of the flip-flops, unless the flip-flops are preset by other means. A pin on an integrated circuit may be used as a reset terminal for receiving the reset signal. However, in order to increase the integration degree of an integrated circuit, pins are used primarily as terminals connected to a power supply or signal lines, and it is desired not to use the pins for other terminals, such as the reset terminal for receiving the reset signal. In order not to use a pin for a reset terminal, it is necessary to assemble an output circuit for providing the reset signal in an integrated circuit whereby the reset signal is outputted automatically when the electric potential of a power supply rises at the time of turning on the power supply.